Calibration (density correction) plays a critically important role in adjusting color in a color-capable image forming apparatus utilizing an electrophotographic process such as a copier, a printer, a facsimile machine or the like. Thus, an improvement in its correction accuracy has been strongly demanded. However, frequent halts of the apparatus at every execution of the calibration leads to confusion in prioritizing. A reduction in printing efficiency (throughput) of the apparatus due to the calibration must be suppressed as much as possible.
As an approach to meeting such a demand, an art has been known in which as soon as detecting that the number of sheets printed and the remaining number of sheets to be printed exceeds a predetermined number of sheets during continuous printing, a detection image generating section forms three kinds of detection images by toner images with each color of cyan (C), magenta (M), yellow (Y) and black (B), for example, at a 30% density, a 50% density and a 70% density as made into a unit of CMYB, respectively, in a region on a conveying belt between a preceding sheet and a subsequent sheet after the predetermined number of sheets. The detection images are detected by a reflective photosensor. A difference in density is compared with an ideal density value, and a density correction is carried out if necessary (see Japanese Published Unexamined Patent Application No. 2006-79001). According to the art described in the foregoing patent literature, printing does not have to be interrupted during the toner density adjustment, thereby allowing printing efficiency to be improved.
However, in the prior art described in the aforementioned patent literature wherein three kinds of patch images varying in density ratio (density percentage) are formed to be compared with an ideal density value thereupon conducting a density correction when needed, developing characteristics of a printer engine cannot be corrected, and merely input/output characteristics (image processing) are changed. Therefore, there is a possibility that when relatively large color fluctuations occur during continuous printing, the fluctuations cannot be suppressed.